1. Field of the Invention
The present invention relates to RF and microwave mixers and more particularly to mixer topologies configured to cancel low order spurious output signals (also known as spurs).
2. Description of the Prior Art
Mixers are generally known in the art and are used in various applications for upconverting or downconverting microwave and RF signals having a frequency f1 to a higher or lower frequency for by way of a local oscillator. More particularly, such mixers are non-linear devices with two input ports and one output port. One input port is used for receiving microwave or RF input signals having a frequency f1 while the other input port is for a local oscillator signal having a frequency f2. When signals having frequencies f1 and f2 are applied to the input ports, the following signals are generated at the output port: the original signals, f1, f2; harmonics of the signals 2 f1 and 2 f2, etc; the sum and differences of the signals f1 and f2; as well as the sum and differences of each of the harmonics of the signals f1 and f2. In general, the output signals available at the output of a mixer are provided by equation (1) below:foutput=±M* f1±N* f2, where M and N are integers and the sum |M|+|N|=“order” of the mixer output signal frequency.  (1)
The mixer output signals are normally identified by their respective coefficients. For example, a spur at 2 f1+2 f2 is identified as (2, 2). Similarly, the spur at 2 f1+f2 is identified as (2, 1). The signal f1 is identified as (1, 0) while the signal f2 is identified as (0, 1). The harmonics of these signals may are also identified using the coefficient notation. For example, the spur at 2 f2 is identified as (0, 2) while the spur 2 f1 is identified as (2, 0).
When the mixer is used as an upconverter, the desired output frequency of the mixer is greater than the RF input signal, f1+f2, for example. Similarly, when the mixer is used as a downconverter, the desired output of the mixer is lower than the RF input signal, f1−f2, for example. The balance of the signals available at the output of the mixer are undesirable and are therefore spurious output signals, or simply spurs. Such spurs are well known and relate to the inherent characteristics of the mixers, for example, as disclosed in “Effects of Offsets on Bipolar Integrated Circuit Mixer Even-Order Distortion Terms”, by Coffing et al., IEEE Transactions On Microwave Theory and Techniques, Vol. 49, No. 1, January 2001, pages 23–30, hereby incorporated by reference.
Many of the spurs at the mixer output port can oftentimes simply be filtered out with simple low pass or band pass filters. In addition, the power level of many of the spurs decreases the further the spur frequency is away from the desired output frequency. Thus, due to this low power level, many of the spurs are simply ignored. However, spurs which occur in the desired frequency band or close to the frequency band are problematic and cause interference. Various techniques are known to be used to eliminate low order spurs which cause interference. For example, in one known application, the band is split and multiple stage mixers are used. In such an application, the band split is selected to eliminate various low order spurs. For example, in known downconverter applications, the (2, 1) and (2, 2) spurs are known to drive the band splits. In other known applications, the low order spurs are filtered out by relatively complex and expensive narrow band filters. Both of these techniques degrade the overall performance of the system. Thus, what is needed is a mixer topology which cancels out low order spurs in order to eliminate the need for band splitting and relatively expensive and complex narrow band filters.